//Answer:
ΔH₁₊₂₊₃ = -74.6 Kj
Video Lecture I did on these type problems... See attached
Explanation:
The vid starts with some simple Hess Law additions and progresses through to more complex systems. The problem given in your example is a bit complex as a starting point when learning about these systems, so I suggest watching the above video lecture that develops the concept from less complex systems to the more complex.
Never the less, here's your problem, but I suggest you go through the video recommended above to start with some less complex problems.
Target Rxn => C(s) + 2H₂(g) => CH₄(g)
Given:
Rxn 1 => C(s) + O₂(g) => CO₂(g) ΔH₁ = -393.5 Kj
Rxn 2 => CH₄(g) + 2O₂(g) => CO₂(g) + 2H₂O(g) ΔH₂ = -890.8 Kj
Rxn 3 => H₂(g) + 1/2O₂(g) => H₂O(l) ΔH₃ = -285.8 Kj
____________________________________________
Start by adding Rxns 1 & 2, but reverse Rxn 2, change sign of ΔH₂ to +890.8Kj
Rxn 1 => C(s) + O₂(g) => CO₂(g) ΔH₁ = -393.5 Kj
Rxn 2 => CO₂(g) + 2H₂O(l) => CH₄(g) + 2O₂(g) ΔH₂ = +890.8 Kj
Rxn 1,2 => C(s) + 2H₂O(l) => CH₄(g) + O₂(g) ΔH₁₊₂ = +497.3 Kj
Now, Double Rxn 3 and add to Rxn 1,2 => Target Rxn
Rxn 1,2 => C(s) + 2H₂O(l) => CH₄(g) + O₂(g) ΔH₁₊₂ = +497.3 Kj
Rxn 3 => 2H₂(g) + O₂(g) => 2H₂O(l) ΔH₃ = 2(-285.8) Kj = -571.6 Kj
Rxn 1,2,3 => C(s) + 2H₂(g) => CH₄(g)
ΔH₁₊₂₊₃ = (+497.3 Kj) + (-571.6 Kj) = -74.3 Kj.
